Basically the Sliderman started about a year ago during a Let's Play/Livestream of Slender: Sanatorium by DeliciousCinnamon, in one particular scene in the game, the Let's Players looked back for a moment to see Slender Man sliding across the room, and the viewers as well as the Let's Players found it hilarious.
Needless to say, a week or so after the Livestream, a deviantART user named MoonHitler created this flash game titled "Sliderman" as a tribute:
The game starts with slightly unnerving music while "Sliderman" is shrouded in blackness and then the game starts with "Sliderman" sliding across rainbows with the Mario 64 slide music playing, the object is to keep "Sliderman" on the rainbows until you fall off the rainbows and hope to get a good score, the controls are incredibly simple, you click play and then you use you space key to make "Sliderman" jump.
Also, if you lose, you can click the text "Lex Wins" (a reference to another DeliciousCinnamon meme) and then replay the game you can now jump on the clouds as well as having the ability to make "Sliderman" double-jump.
The video that started it all (The "Sliderman" incident occurs at 4:49):
My Highest Score Was 33, What Was Yours?